The invention relates to a process or method for producing an iron-containing sorption material for treating water or an aqueous effluent. An iron-containing solution is adjusted to a specific salt content and thereby transformed into an iron oxide-containing suspension. The sorption material is obtained from the solids content of this suspension.
It has been known in water purification the art for a number of years to utilize the adsorption properties of iron hydroxide in technical systems. For this purpose, substances containing iron as metal or oxide or hydroxide are used in the form of granulates. These granulates are used as a filter bed in simple filter cells or special reactors. The water or effluent to be purified is guided through the filter bed to remove dissolved harmful trace constituents from the water or effluent through adsorption to the surface of the material.
Furthermore, methods are known in the art in which precipitation processes are used for the same purpose and a dissolved iron compound is added to the water or effluent. By subsequent hydrolysis, iron hydroxide floc is formed to which the pollutants are adsorbed. The floc is then separated from the water or effluent. This requires substantial control and monitoring, however, to ensure that the purification effect on the water or effluent remains constant.
Published German patent application no. DE 43 20 003 A1, for example, discloses a process for removing dissolved arsenic using solid iron hydroxide for water purification. The iron hydroxide can be obtained from the suspension by adding a stoichiometrically equivalent amount of an alkaline solution to an acid iron salt solution until the pH remains stable in the neutral range between 6 and 8. The suspension is subsequently washed and is then available for the conditioning of fixed bed filters. In addition, after the suspension has been produced, it can be transformed into a hydroxide gel by centrifugation. The gel is then subjected to freeze conditioning at temperatures below xe2x88x925xc2x0 C. to convert the gel into a granulated material, which can be used directly in the fixed bed filters. In practice, a drawback of this process is the poor quality of the granulated material because the material is comparatively soft and fine-grained. As a result, a substantial amount of material is lost when a filter filled with the material is backwashed. New material must therefore be added after each backwashing, which significantly adds to the cost of the process. In addition, the water or effluent to be treated must be relatively clean. Otherwise, solids in the water are adsorbed on the granulate because of the filtering action. This interferes with the removal of the desired components.
Published German patent application no. DE 197 45 664 A1 discloses a known process for producing a granulate for purifying an arsenic-containing fluid. The granulate is essentially produced from silica sand and iron powder or iron chips by calcining at high temperatures. In the subsequent process for arsenic removal, this granulate must first be activated by dissolved oxygen to bring about an oxidation through the reaction with the oxygen and the water in which iron hydroxide is formed. In this oxidized form, arsenic can be adsorbed to the granulate to remove the undesirable arsenic from the water or effluent. A drawback of this process is the high cost of energy used in production of the sorbent material caused particularly by the high process temperatures. Furthermore, the water or effluent to be treated is modified by the reaction of the metallic iron forming the iron hydroxide, as this binds oxygen from the water and lowers the pH of the water through the release of H+ ions. Because of the small amounts of iron contained in the material, the absorption capacity of the material is moreover limited. In addition, a relatively long contact time between water and granulate is required. As a result, the corresponding filter systems must be very large and are therefore uneconomical.
Published German patent application no. DE 198 34 916 A1 discloses a filter paper for water purification, which contains iron oxide and iron hydroxide, among other things, and which can be used, for example, to purify arsenic-containing water. Described, for instance, is the single or multiple charging of the filter paper with the contaminated water or the use of pieces of filter paper in reactors or filter cells. A drawback in practice is that technical implementation can be connected with significant problems because the pressure of the water can cause the pieces of filter paper to stick together and to form a dense and poorly permeable filter body.
It is an object of the invention to provide an improved process for preparing an iron-containing sorption material.
Another object of the invention is to provide an efficient, low-cost process for preparing an iron-containing sorption material.
A further object of the invention is to provide a process for producing a particulate sorption material which has a high degree dimensional stability and abrasion resistance.
An additional object of the invention is to provide a process for producing an iron-containing sorption material which is simple and economical in use with simultaneous high efficiency.
It is also an object of the invention to provide a process for producing an iron-containing sorption material which does not require activation prior to use.
These and other objects are achieved in accordance with the present invention by providing a process for producing an iron-containing sorption material for treating water or effluent, said process comprising adjusting the salt content of an iron salt-containing solution to transform the solution to an iron oxide-containing suspension, and treating the iron oxide-containing suspension to obtain a particulate iron oxide-containing sorption material from suspended iron-containing solids in the suspension, wherein said treating comprises subjecting the iron-containing solids to a relative pressure increase of the suspension at a temperature of less than 5xc2x0 C., whereby the solids from the suspension are formed into solid complexes.
Further advantageous embodiments of the process according to the invention are described hereinafter.
According to the invention, a method is provided in which iron-containing solids are formed into solid complexes by a relative pressure increase of the suspension at a temperature of less than 5xc2x0 C. With this method, a sorption material is provided whose solid complexes have a high degree of dimensional stability. As a result, under the influence of high fluid pressure, they do not tend to stick together or clog the gaps. This makes it possible, in particular, to use the sorption material to treat even highly polluted effluents without risking an efficiency-reducing adsorption of dirt particles contained in the effluent. Furthermore, in the production process it is not necessary to activate the sorption material prior to use. This facilitates the use of the sorption material on the one hand and excludes undesirable influences on the quality of the water or effluent on the other, especially a change in the pH. Furthermore, the use of the sorption material produced in accordance with this process clearly reduces the control and monitoring costs, as the sorption material does not need to be monitored. In addition, the sorption material produced by this process has a high iron oxide or iron hydroxide content because the use of fillers and auxiliary agents is largely avoided. In practice, the solid complexes have a diameter of between 0.1 and approximately 3 mm.
A convenient and effective way of subjecting the suspension to an increase in pressure is to subject the suspension to a freezing temperature, i.e., to a temperature of less than 0xc2x0 C., preferably less than xe2x88x925xc2x0 C., in a sealed container. In this way, the well known expansion of aqueous systems as they freeze serves to apply pressure to the suspension.
In one particularly advantageous embodiment of the process according to the invention, the solid complexes are formed or shaped into dimensionally stable grains by the pressure increase. This substantially facilitates the handling and thus the industrial use of the material and thereby increases the economic efficiency. By controlling the pressure pattern or the maximum pressure, it is possible specifically to influence the quality of the grains so that they can be adapted to the individual application conditions.
It is particularly advantageous if the pressure is increased within a pressure range of between 4*107 Pa and 108 Pa. In practice it has been shown that in addition to the iron hydroxide particles combining into dimensionally very stable iron hydroxide grains, crystallization processes also take place, which further improve the physical properties of the iron hydroxide grains.
In a particularly advantageous embodiment, the solids content of the suspension is at least 10%. This makes it possible to prepare solid complexes that are highly reactive and, in addition, have the desired mechanical properties that make them suitable for a broad range of industrial applications. Solids contents of 15% to 25% are optimally suited for economically efficient applications.
An especially effective embodiment of the process according to the invention is achieved if the pressure is released after it has previously been increased to enhance the dimensional stability of the solid complexes. By controlling the pressure release, it is possible specifically to obtain the desired geometries and surface properties of the solid complexes. The pressure release is particularly related to the preceding pressure increase so that it is determined independently of the ambient pressure.
Another especially advantageous embodiment of the process according to the invention is achieved if the solid complexes are dried. As a result, the sorption material can be used as intended directly after the production process. This prevents, in particular, any accidental deformation of the solid complexes due to undesirable environmental influences after the pressure-dependent shaping.
It is especially advantageous if the solid complexes are dried by heating to remove the moisture quickly. In practice, it has been observed that this improves the solid complexes. The background is again a regrouping of the components of the solid complexes, the pattern of which can be substantially determined by controlling the temperature influences. It is particularly practical if air is introduced for drying. This reduces the cost of energy in production. Also feasible are modifications where the gaseous medium used for drying can in turn be used as a carrier for reactive substances.
In a further particularly advantageous embodiment of the method according to the invention, the solid complexes are treated with a cation-containing activating solution. This changes the polarization on the surface of the solid complexes, making it possible to further enhance the process of adsorbing the undesirable pollutants from the water or effluent.